1. Field of the Invention
The invention relates to a pushbutton device for mechanical preselection tuning in radio receivers, and more particularly to a device comprising a mainly rectangular adjusting member which is rotatable about its longitudinal axis and which is coupled to the tuning means of the receiver. Such a member can be driven, by way of a slip coupling, by a tuning control or knob and, moreover, directly by cooperation with preselection discs which are adjustably and fixably provided on the slide rods of the pushbuttons, the adjusting member and the slip coupling constituting an elongate unit which is rotatable as one assembly and which is journalled in the frame of the receiver near its two ends.
Preselection tuning is effected by depressing one of the buttons so far that the straight edge of the associated, approximately semi-circular preselection disc abuts against the adjusting member and forces the latter to take over the rotary position of the disc which corresponds to a preselected station. Thanks to the presence of the slip coupling between the adjusting member and the tuning knob with associated gearwheel or worm wheel transmission, the rotation of the adjusting member takes place without the elements of the transmission being rotated.
2. Description of the Prior Art
In known devices of this kind (see for example, German Gebrauchsmuster No. 1,835,052), the adjusting member has rigidly connected thereto one or two shaft stubs which are journalled in the frame of the receiver, the slip coupling, usually consisting of a disc and a gearwheel or a worm wheel, being arranged on one of these shaft stubs. The moment of slip -- i.e. the turning moment at which the coupling starts to slip, which is also the largest moment which can be transmitted -- should be sufficient to overcome, when the tuning control is turned, the friction in the two bearings of the adjusting member and the friction of the tuning means -- usually slide cores -- driven by this member. On the other hand, the moment of slip should be as small as possible because, when a pushbutton is depressed, the tuning knob is stationary and -- besides the said frictions -- the moment of slip must be overcome.